The present invention relates to high density fused silica mixes that can be formed by impact presses into high density refractory shapes.
Fused silica refractories are mainly used in industrial processes where temperatures generally do not exceed 2000.degree. F. and/or the refractory is exposed to severe thermal cycling. Fused silica refractories are also used in acidic environments which generally operate at elevated temperatures. Examples of fused silica application areas are coke ovens, chlorinators, ceramic kilns, molten metal shrouds, and the like.
Up to the present it has not been possible to make pressed fused silica shapes such as brick, plates, or the like, having acceptable densities and strengths by power pressing. As a consequence it has been necessary to utilize slip casting to make fused silica shapes having densities from 117 to 122 pcf. Such density and also high strengths are required since such shapes are used in areas such a chlorinators and other application where low porosity, low permeability, and acid resistance are of paramount importance.
While slip casting is generally satisfactory to make fused silica shapes, it is a costly process by virtue of the long processing time and low production rates. More particularly, forming shapes by the slip method requires long residence times in plaster molds and long drying times before the shape can be fired. As a result the production costs for making shapes such as brick or plates by slip casting is much higher than utilizing power pressing techniques commonly used in forming brick.
The reason for the long drying times is obvious since in order to be utilized in a slip, as opposed to being power pressed, for any given formulation there is at least about four times more water in a slip to be used as opposed to a composition that is to be power pressed.
Efforts to try to use a press to form a silica brick of high amorphous content is set forth in the McCreight et al Patent, U.S. Pat. No. 3,463,650 in which vitreous silica grain made according to the teachings of U.S. Pat. No. 3,314,758 was used in a brick mix. This mix contained fumed silica and impure binders which caused devitrification (formation of crystalline phases) of amorphous silica. These bricks were characterized by a bulk density of 115-116 psi and a strength of 700-800 psi as measured by room temperature modulus of rupture. Although this patent shows that brick could be made in the laboratory with an amorphous content of 95%, in reality, brick fired in production quantities required a longer hold time for the mass to reach equilibrium and amorphous contents of 80 to 90% were more typical. These brick had inferior thermal shock resistance compared to 100% amorphous silica and at times the strengths are less than 700 psi.
For a long time it has been desired to produce a fused silica brick. by power pressing which has more than 90% amorphous content, a density near or above 120 pcf, and a strength over 1000 psi.
Thus, the McCreight patent which issued some twenty-four years ago showed an effort to try to eliminate slip casting which has not been successful and the more recent Dunworth Patent No. 5,096,865 shows the continued use of slip casting in order to form high density fused silica shapes.